1 / 17

Small, fast, low-pressure gas detector

Small, fast, low-pressure gas detector. E. Norbeck, J. E. Olson, and Y. Onel University of Iowa For DNP04 at Chicago October 2004. Typical low-pressure PPAC. ( P arallel P late A valanche C ounter). Two flat plates Separated by1-3 mm Filled with 10-80 torr isobutane

giovanna-abbott
Télécharger la présentation

Small, fast, low-pressure gas detector

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Small, fast, low-pressure gas detector E. Norbeck, J. E. Olson, and Y. Onel University of Iowa For DNP04 at Chicago October 2004

  2. Typical low-pressure PPAC (Parallel Plate Avalanche Counter) • Two flat plates • Separated by1-3mm • Filled with 10-80 torr isobutane • 500-1000 V between plates DNP04 BB.014 Gas Detector

  3. Small PPAC for showers from high-energy (10-1000 GeV) electrons • The original object of this study was to determine the suitability of a PPAC as an inexpensive, very fast, rad-hard pixel detector to use in a calorimeter for electrons. • Our measurements have broader application. DNP04 BB.014 Gas Detector

  4. Single Pixel PPAC For Test With High-Energy Electrons • Gap 1.0 mm • Cathode 7X0 = 29 mm of tantalum • Area of anode is 1.0 cm2 • Guard ring to simulate neighboring pixels • Gas is isobutane at 10 to 100 torr DNP04 BB.014 Gas Detector

  5. Detail of 1 mm gap and guard ring DNP04 BB.014 Gas Detector

  6. A MIP will usually leave no ionization in the low pressure gas. With a high-energy electron shower there are 100s or 1000s of electrons contributing to the signal. • To date we have not yet put a high-energy electron into the detector. • Our measurements have all been with Compton electrons from a 137Cs gamma source. With the source to the side of the PPAC, a few of the electrons travel parallel to the face of the plates and produce a usable amount of ionization in the gas. DNP04 BB.014 Gas Detector

  7. 1.8 ns 50 torr 790 V 7 mv into 50  Electron signal Single peak with considerable noise. The noise is large because of the small size of the signal using our 137Cs source. With the much larger signals from high-energy electrons, the noise will be negligible. DNP04 BB.014 Gas Detector

  8. For high speed, the RC time constant must be kept small. Only PPACs of small area are fast ~1 ns R = 50 Ω (coax cable). C is the capacity between the plates C = .885 pF for 1 mm gap and area of 1 cm2 For our larger PPAC with C = 168 pF rise time ~5 ns fall time ~7 nsFast enough for a Zero Degree Calorimeter at the LHC where minimum beam crossing time is 25 ns. DNP04 BB.014 Gas Detector

  9. .3 ms 0.5 ms 50 torr 790 V Ion collection time DNP04 BB.014 Gas Detector

  10. Reflections are a problem with such fast signals. Should be 50 Ω all the way to the anode. Guard ring View with covers removed Signal out DNP04 BB.014 Gas Detector

  11. At isobutane pressures less than 30 torr afterpulses sometimes occur during the first 20 ns. This is a worst case example. Total charge from the afterpulses can be much larger than primary signal. 10 torr 500 V DNP04 BB.014 Gas Detector

  12. The afterpulses seen here are usually hidden inside of signals that are more than 20 ns wide. This may be the cause of the typically bad energy resolution of PPACs operated in the 5 to 20 torr range. What causes the afterpulses? They are most likely caused by UV photons producing photoelectrons at the cathode. These electrons then initiate a new avalanche. Changing the anode from stainless steel to graphite had no effect on the afterpulses. This shows that the photons do not come from the anode. DNP04 BB.014 Gas Detector

  13. Perhaps the excited molecules emit photons with a lifetime long compared with 20 ns, with molecular collisions limiting the lifetime of the excitations. Collision time in isobutane gas is too long to account for the data. Isobutane speed 350 m/sFragments are fasterIon speed > 2000 m/s (1 mm in 500 ns) Note also that electrons acquire a larger energy between collisions at the lower gas pressures. 500 V at 10 torr but 1000V at 80 torr DNP04 BB.014 Gas Detector

  14. Ion current from same event Afterpulses are real avalanches DNP04 BB.014 Gas Detector

  15. The area under the ion peak is clearly larger than the area under the electron peak. The signal is caused by the motion of the charges in the 1 mm gap (not by the collection of the charges). Most of the charges generated by the avalanche are produced close to the anode so that electrons move only a short distance, while the ion move almost the entire millimeter. Signal processing can easily remove the slow ion peak form the signal. DNP04 BB.014 Gas Detector

  16. PPAC can be made resistant to radiation damage • The walls and electrodes can be made of durable metal in high-energy applications. • A single spark can make a sharp point on the metallic surface of the cathode that will make the PPAC inoperable. The energy carried by a spark must be kept small, and provision must be made to keep sparking to a minimum. • Aging (polymerizing of the gas) must be prevented. (Low pressures and short distances require special considerations.) DNP04 BB.014 Gas Detector

  17. Conclusions • Small area PPACs can be made to be radhard and fast ~ ns. • PPACs have been used for 30 years, but more research is still needed maximize their potential. DNP04 BB.014 Gas Detector

More Related